This invention relates to a pile and a method of installing a pile. More especially, but not exclusively the invention relates to piles for moorings for floating structures such as offshore oil installations and vessels.
Known anchoring systems include driven piles, suction anchors, drag embedment anchors and vertically loaded anchors and conventional drilled piles. All have disadvantages:
Driven piles must be of heavy construction since they are hammered into the ground or seabed. They are additionally not suitable for all kinds of ground.
Suction anchors are of limited use in hard soils such as coral or compacted clay. They are expensive. After use because they are above the mud-line they must generally, be recovered which adds to the cost.
Drag embedment anchors require high pre-tensioning to ensure correct embedment. In deep water this is hard to achieve without a tenisioning device. Tensioning devices add to the complexity and cost of the operation. Additionally drag embedment anchors accept only small vertical forces.
Vertically loaded anchors are difficult to embed and require a drag force of about 50% of the ultimate load capacity. This can be hard to achieve in deep water.
Conventional drilled piles are expensive since they are time consuming to install
U.S. Pat. No. 3,934,528 (Deep Oil Technology Inc.) describes an offshore tension leg platform. Lengths of drill pipe may be connected together and extend through an annular casing received in a buoyant support member. The lengths of drill pipe can be manipulated by a power swivel and winch. The string of drill pipe can be used to introduce ballast to or remove it from an anchor member on the seabed. Once the anchor is ballasted in position a pile may be installed by conventional drilling and cementing. The drill pipe, swivel and winch can be used for this.
The invention seeks to overcome or reduce the problems associated with the prior art. According to the invention there is provided a method of drilling a pile in ground comprising the steps of:
i. providing a pile,
ii. providing a drill bit at an end of the pile rotatable relative to the pile;
iii. engaging the ground with the drill bit; and
iv. rotating the the bit relative to the ground and the pile generating a hole into which the pile is received.
According to the invention there is further provided a pile having provided one end thereof with a drill bit rotatable relative to the pile.
The invention can be relatively quick and inexpensive to install since it can be a one trip process; drilling and insertion occur in the same process. At least some embodiments of the invention provide a pile system for example for moorings which may be drilled to its design depth without the need for pre-drilled hole or for retraction and re-insertion of the pile during installation. The pile is drilled by rotating a drilling bit relative to the ground while restraining, generally the pile as a whole from rotation. Rotary motion may be transmitted to the drill bit by rotating an elongate member received in the pile. Bearings may be provided to aid this. The elongate member may be connected to a non-recoverable drilling bit of a diameter greater than the pile for example by a drive spline. The elongate member may be conduit supplying fluid to a downhole motor Some of the components such as the elongate member and motor or turbine may be recovered following deployment. Instead of using a downhole motor the elongate member may be driven from an installation vessel for example by a rotary motor. In some embodiments of the invention the drill bit may drill a hole of greater diameter than the pile. This can be achieved using, bi-centred, jetting bits or under-reamers (or other collapsible bits) which can be retrieved Alternatively a hole of a diameter less that the pile could be drilled, allowing recovery of the bit; embedment being achieved either by relying on fluid erosion to create a diameter large enough to allow the pile to advance or by relying on applied weight to displace soft sediments. This is of particular application where it is desired to grout the pile into the hole. Grouting may be undertaken even if oversize bits are not employed. Grouting can be achieved in conventional way or by using a cement fill-up device to divert slurry into cement hoses which are directed to an annular gap. The mooring line, parts or terminations thereof can be pre-installed prior to deployment of the pile. If desired a linkage point such as a mooring line termination can be mounted on a bearing assembly allowing the linkage to swivel to align itself to applied tension thereby avoiding the need to orient the pile with respect to the anticipated load to maintain its efficiency. If desired the pile can be oriented with respect to the anticipated load. If desired the pile may comprise a nest of concentric members coupled together for example with cement. This can provide a cheap high strength pile especially where the concentric members are made from standard oil field casing. The invention may be installed in the seabed utilising a vessel without using a rigid, tubular conduit. This allows the use of a (low cost) barge rather than an (expensive) floating drilling unit. This may be achieved by suspending the pile from a flexible member such as a crane line and driving the bit by a downhole motor connected by a hose to a fluid supply on the barge. In some embodiments of the invention fins which may be fixed or movable axially are provided on the pile. They resist reaction forces attempting to rotate the pile generated by the motor and allow the pile to be drilled when suspended from a member which is not torsionally rigid such as a crane wire.
Rotation of the bit may be achieved by rotation of the elongate member using rotary transmission means of an installation vessel.
Where a downhole motor is provided means for decoupling and recoupling it in situ may be provided.